Light-guide optical element employing complementary coated partial reflectors, and light-guide optical element having reduced light scattering

ABSTRACT

A transparent substrate has two parallel faces and guides collimated image light by internal reflection. A first set of internal surfaces is deployed within the substrate oblique to the parallel faces. A second set of internal surfaces is deployed within the substrate parallel to, interleaved and in overlapping relation with the first set of internal surfaces. Each of the internal surfaces of the first set includes a first coating having a first reflection characteristic to be at least partially reflective to at least a first subset of components of incident light. Each of the internal surfaces of the second set includes a second coating having a second reflection characteristic complementary to the first reflection characteristic to be at least partially reflective to at least a second subset of components of incident light. The sets of internal surfaces cooperate to reflect all components of light from the first and second subsets.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 62/943,867, filed Dec. 5, 2019, whose disclosure isincorporated by reference in its entirety herein.

TECHNICAL FIELD

The present invention relates to display systems and in particularlight-guide optical elements suitable for use in a display.

BACKGROUND OF THE INVENTION

Certain display technologies, particularly suitable for head-up displays(HUDs) such as near-eye displays (NEDs) for virtual reality andaugmented reality applications, employ a light-guide optical element,also referred to as an “optical waveguide” or a “light-transmittingsubstrate”, with a series of internal oblique mutually parallelpartially reflective surfaces. An image projector is optically coupledto the waveguide and injects light corresponding to a collimated imageinto the waveguide so as to propagate through the waveguide by internalreflection. The propagating light is progressively coupled out of thewaveguide towards an observer's eye by reflection at the series ofpartially reflective surface, thereby expanding the effective opticalaperture opposite the eye compared to the output aperture of the imageprojector.

Reflectivity of the partially reflective surfaces is sensitive tovarious parameters of the incident light, including the spectral range,polarization direction, and angle of incidence. The partially reflectivesurfaces are typically coated with optical coatings to generate adesired reflectivity pattern.

SUMMARY OF THE INVENTION

The present invention is a light-guide optical element.

Certain preferred embodiments according to one aspect of the presentinvention provide a light-guide optical element having internal partialreflectors coated according to an optical coating scheme that enablessimultaneous satisfying of spectral, polarization and angular uniformityrequirements. In other embodiments of this aspect of the presentinvention, the aforementioned requirements are satisfied whilesimultaneously minimizing reflections in undesired directions. Certainpreferred embodiments according to another aspect of the presentinvention provide a light-guide optical element having an amount of areflection suppressing material applied to one or more regions of anexternal surface or surfaces of the light-guide optical element, thatreduces light scattering within the light-guide optical element.

According to the teachings of an embodiment of the present invention,there is provided an optical device. The optical device comprises: alight-transmitting substrate having at least two parallel major externalsurfaces for guiding light indicative of a collimated image by internalreflection at the major external surfaces; a first set of mutuallyparallel internal surfaces deployed within the substrate oblique to theexternal surfaces; and a second set of mutually parallel internalsurfaces deployed within the substrate parallel to, interleaved with andin overlapping relation with, the first set of internal surfaces, atleast part of each of the internal surfaces of the first set including afirst coating having a first reflection characteristic so as to be atleast partially reflective to at least a first subset of components ofincident light, and at least part of each of the internal surfaces ofthe second set including a second coating having a second reflectioncharacteristic, that is complementary to the first reflectioncharacteristic, so as to be at least partially reflective to at least asecond subset of components of incident light, such that the sets ofinternal surfaces cooperate to reflect all components of light from thefirst and second subsets.

Optionally, the first subset of components includes light correspondingto a first color, and the second subset of components includes lightcorresponding to a second color.

Optionally, the first subset of components includes light having a firstpolarization direction, and the second subset of components includeslight having a second polarization direction.

Optionally, at least one of the first or second coatings includes astructural polarizer.

Optionally, at least one of the first or second coatings includes adielectric coating.

Optionally, at least one of the first or second coatings includes ametallic coating.

Optionally, the first coating is configured to: reflect light havingwavelengths corresponding to a first color with a first reflectionefficiency, reflect light having wavelengths corresponding to a secondcolor with a second reflection efficiency, and reflect light havingwavelengths corresponding to a third color with a third reflectionefficiency less than the first reflection efficiency, and the secondcoating is configured to reflect light having wavelengths correspondingto the first color with a reflection efficiency that is greater than thethird reflection efficiency, such that the combined reflectionefficiency of the third color by the first and second coatings isgreater than or equal to the first reflection efficiency.

Optionally, the second reflection efficiency is less than the firstreflection efficiency, and the second coating is configured to reflectlight having wavelengths corresponding to the second color with areflection efficiency that is greater than the second reflectionefficiency, such that the combined reflection efficiency of the secondcolor by the first and second coatings is greater than or equal to thefirst reflection efficiency.

Optionally, the second coating is configured to reflect light havingwavelengths corresponding to the first color with a reflectionefficiency that is approximately equal to the first reflectionefficiency.

Optionally, the first coating is configured to: reflect light havingwavelengths corresponding to a first color with a first reflectionefficiency, reflect light having wavelengths corresponding to a secondcolor with a second reflection efficiency less than the first reflectionefficiency, and reflect light having wavelengths corresponding to athird color with a third reflection efficiency less than the firstreflection efficiency, and the second coating is configured to: reflectlight having wavelengths corresponding to the first color at areflection efficiency greater than the second and third reflectionefficiencies, reflect light having wavelengths corresponding to thesecond color at a reflection efficiency greater than the second andthird reflection efficiencies, and reflect light having wavelengthscorresponding to the third color at a reflection efficiency greater thanthe second and third reflection efficiencies.

Optionally, the first coating includes a patterned coating comprising anumber of portions of a reflective material arranged on each of theinternal surfaces of the first set in a prescribed pattern.

Optionally, each portion of the reflective material has a circular shapein a plane of the internal surfaces.

Optionally, each portion of the reflective material has an oblong shapein a plane of the internal surfaces.

Optionally, the reflective material is a dielectric material.

Optionally, the reflective material is a metallic material.

Optionally, spaces formed between the portions of the reflectivematerial are transparent.

Optionally, a second reflective material is deployed on the internalsurfaces in spaces formed between the portions of the reflectivematerial.

Optionally, the second reflective material includes a dielectricmaterial.

Optionally, the second reflective material is arranged on the internalsurfaces in a prescribed pattern.

Optionally, at least one of the number of portions or a size of theportions on the internal surfaces of the first set increases withrespect to a primary direction of propagation of light through thesubstrate.

Optionally, the optical device further comprises an amount of a lightreflection suppressing material deployed between the reflective materialand at least part of the internal surfaces of the first set.

Optionally, the light reflection suppressing material includes a lightabsorbing material.

Optionally, the light reflection suppressing material includes a lightscattering material.

Optionally, the first coating is deployed on a first portion of each ofthe internal surfaces of the first set, and the second coating isdeployed on a second portion of each of the internal surfaces of thefirst set, and the second coating is deployed on a first portion of eachof the internal surfaces of the second set, and the first coating isdeployed on a second portion of each of the internal surfaces of thesecond set, and the first and second portions of the internal surfacesof the first set are non-overlapping portions, and the first and secondportions of the internal surfaces of the second set are non-overlappingportions.

Optionally, the internal surfaces of the first and second sets reflect aproportion of light, guided by internal reflection at the major externalsurfaces, out of the substrate toward an eye of a viewer.

Optionally, the internal surfaces of the first and second sets reflect aproportion of light, guided by internal reflection at the major externalsurfaces, out of the substrate so as to be coupled into a secondlight-transmitting substrate for guiding by internal reflection atexternal surfaces of the second light-transmitting substrate.

Optionally, the substrate is configured to guide light in one dimensionthrough the substrate.

Optionally, the substrate is configured to guide light in two dimensionsthrough the substrate.

Optionally, at least one of the internal surfaces from at least one ofthe first or second sets includes an end region associated with a firstof the external surfaces of the substrate defining an interface regionbetween the at least one internal surface and the substrate, and thefirst of the external surfaces has an amount of light absorbing materiallocated in an indentation formed in the first of the external surfacesat the interface region.

There is also provided according to an embodiment of the teachings ofthe present invention an optical device. The optical device comprises: alight-transmitting substrate having at least two parallel major externalsurfaces for guiding light indicative of a collimated image by internalreflection at the major external surfaces; and a plurality of mutuallyparallel internal surfaces deployed within the substrate oblique to theexternal surfaces, at least part of a first subset of the internalsurfaces comprising a patterned coating that includes a number ofportions of a reflective material arranged on the internal surfaces ofthe first subset in a prescribed pattern, the patterned coating being atleast partially reflective to at least a first subset of components ofincident light, a second subset of the internal surfaces being at leastpartially reflective to at least a second subset of components ofincident light, and the internal surfaces of the first subset being inoverlapping relation with the internal surfaces of the second subsetsuch that the subsets of internal surfaces cooperate to reflect allcomponents of light from the first and second subsets.

Optionally, each portion of the reflective material has a circular shapein a plane of the internal surfaces of the first subset.

Optionally, each portion of the reflective material has an oblong shapein a plane of the internal surfaces of the first subset.

Optionally, the reflective material is a dielectric material.

Optionally, the reflective material is a metallic material.

Optionally, spaces formed between the portions of the reflectivematerial are transparent.

Optionally, a second reflective material is deployed in spaces formedbetween the portions of the reflective material.

Optionally, the second reflective material includes a dielectricmaterial.

Optionally, the second reflective material is arranged on the internalsurfaces of the first subset in a prescribed pattern.

Optionally, at least one of the number of portions or a size of theportions on the internal surfaces of the first subset increases withrespect to a direction of propagation of light through the substrate.

Optionally, the optical device further comprises an amount of a lightreflection suppressing material deployed between the reflective materialand the internal surfaces of the first subset.

Optionally, the light reflection suppressing material includes a lightabsorbing material.

Optionally, the light reflection suppressing material includes a lightscattering material.

Optionally, the internal surfaces of the first subset are interleavedwith the internal surfaces of the second subset.

Optionally, surfaces of the first subset of internal surfaces arecoplanar with surfaces of the second subset of internal surfaces.

Optionally, the internal surfaces reflect a proportion of light, guidedby internal reflection at the major external surfaces, out of thelight-transmitting substrate toward an eye of a viewer.

Optionally, the internal surfaces reflect a proportion of light, guidedby internal reflection at the major external surfaces, out of thelight-transmitting substrate so as to be coupled into a secondlight-transmitting substrate for guiding by internal reflection atexternal surfaces of the second light-transmitting substrate.

Optionally, the substrate is configured to guide light in one dimensionthrough the substrate.

Optionally, the substrate is configured to guide light in two dimensionsthrough the substrate.

Optionally, at least one of the internal surfaces includes an end regionassociated with a first of the external surfaces of the substratedefining an interface region between the at least one internal surfaceand the substrate, and the first of the external surfaces has an amountof light absorbing material located in an indentation formed in thefirst of the external surfaces at the interface region.

There is also provided according to an embodiment of the teachings ofthe present invention an optical device. The optical device comprises: alight-transmitting substrate having at least two parallel major externalsurfaces for guiding light by internal reflection at the major externalsurfaces; at least one at least internal surface deployed within thesubstrate oblique to the external surfaces, the internal surface havingan end region associated with a first of the external surfaces of thesubstrate defining an interface region between the internal surface andthe substrate; and an amount of a light absorbing material located in anindentation formed in the first of the external surfaces at theinterface region.

Optionally, the at least one internal surface includes a plurality ofmutually parallel partially reflective surfaces.

Optionally, the at least one internal surface is configured to couplelight, guided within the substrate by internal reflection, out of thesubstrate.

Optionally, the at least one internal surface is configured to couplelight into the substrate so as to propagate within the substrate byinternal reflection.

Optionally, the at least one internal surface is configured to couplelight, guided within the substrate by internal reflection, into a secondlight-transmitting substrate so as to propagate within the secondsubstrate by internal reflection.

Optionally, the light absorbing material includes black absorbing paint.

Optionally, the amount of light absorbing material is sufficient to fillthe indentation.

Optionally, the internal surface has a second end region associated witha second of the external surfaces of the substrate defining a secondinterface region between the internal surface and the substrate, and theoptical device further comprises: an amount of a light absorbingmaterial located in an indentation formed in the second of the externalsurfaces at the second interface region.

There is also provided according to an embodiment of the teachings ofthe present invention method of fabricating an optical device. Themethod comprises: obtaining a light-transmitting substrate having atleast two parallel major external surfaces for guiding light by internalreflection at the major external surfaces, the substrate having at leastone at least internal surface deployed between the external surfaces andoblique to the external surfaces, the internal surface having an endregion associated with a first of the external surfaces of the substrateto define an interface region between the internal surface and the firstof the external surfaces; and depositing an amount of a light absorbingmaterial in an indentation formed in the first of the external surfacesat the interface region.

Optionally, the depositing the amount of the light absorbing materialincludes applying the light absorbing material to substantially theentirety of the first of the external surfaces.

Optionally, the method further comprises: polishing the first of theexternal surfaces to remove the light absorbing material fromsubstantially all portions of the first of the external surfaces thatare outside of the indentation.

Optionally, the obtaining the light-transmitting substrate includes:attaching together a set of coated transparent plates to form a stack,slicing the stack diagonally to form the substrate having the at leasttwo parallel major external surfaces and the internal surface oblique tothe external surfaces, and polishing the external surfaces.

Optionally, the polishing the external surfaces causes the indentationto form in the first of the external surfaces at the interface region.

Optionally, the amount of the light absorbing material is sufficient tofill the indentation.

Optionally, the internal surface has a second end region associated witha second of the external surfaces of the substrate to define aninterface region between the internal surface and the second of theexternal surfaces, and the method further comprises: depositing anamount of a light absorbing material in an indentation formed in thesecond of the external surfaces at the interface region between theinternal surface and the second of the external surfaces.

There is also provided according to an embodiment of the teachings ofthe present invention an optical device. The optical device comprises: alight-transmitting substrate having first and second pairs of parallelmajor external surfaces forming a rectangular cross-section, thesubstrate configured for guiding light by internal reflection at themajor external surfaces; at least one internal surface deployed withinthe substrate oblique to a direction of elongation of the substrateconfigured to couple light out of the substrate; and an amount of alight absorbing material located at a blemish formed at an externalregion of the substrate.

Optionally, the blemish includes a scratch formed in one of the externalsurfaces.

Optionally, the blemish includes a chip in an edge formed between one ofthe external surfaces of the first pair of external surfaces and one ofthe external surfaces of the second pair of external surfaces.

Optionally, the blemish includes a chip in a corner formed between oneof the external surfaces of the first pair of external surfaces and oneof the external surfaces of the second pair of external surfaces.

Optionally, the internal surface includes at least a first end regionassociated with one of the external surfaces of the substrate so as todefine an interface region between the internal surface and thesubstrate.

Optionally, the blemish includes an indentation formed at the interfaceregion.

Optionally, the light absorbing material includes black absorbing paint.

Unless otherwise defined herein, all technical and/or scientific termsused herein have the same meaning as commonly understood by one ofordinary skill in the art to which the invention pertains. Althoughmethods and materials similar or equivalent to those described hereinmay be used in the practice or testing of embodiments of the invention,exemplary methods and/or materials are described below. In case ofconflict, the patent specification, including definitions, will control.In addition, the materials, methods, and examples are illustrative onlyand are not intended to be necessarily limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention are herein described, by wayof example only, with reference to the accompanying drawings. Withspecific reference to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

Attention is now directed to the drawings, where like reference numeralsor characters indicate corresponding or like components. In thedrawings:

FIG. 1 is a schematic side view representation of a light-guide opticalelement (LOE), constructed and operative according to the teachings ofan aspect of the present invention, illustrating the progression ofcomponents of image illumination passing through a sequence ofinterleaved sets of internal partially reflective surfaces havingcomplementary sets of coatings;

FIG. 2 illustrates reflectance curves, as a function of angle ofincidence for p-polarization and s-polarization, of a coating that canbe used in some of the internal partially reflective surfaces;

FIG. 3 is a schematic representation of image illumination componentsreflected by the internal partially reflective surfaces at differentreflected angles;

FIG. 4 is a schematic representation of a patterned reflective coatinghaving portions of reflective material that can be used to compensatefor lower reflection of some of the illumination components;

FIG. 5 is a schematic representation of a patterned reflective coatingsimilar to FIG. 4 , but having portions of reflective material in adifferent shape than the shape of portions of reflective material inFIG. 4 ;

FIG. 6 is a schematic representation of a coating having two reflectivepatterns on a single coating;

FIG. 7 illustrates reflectance curves as a function of wavelength for acoating that can be used in some of the internal partially reflectivesurfaces;

FIG. 8 illustrates reflectance curves achieved when using the coating ofFIG. 7 in combination with a complementary coating used on some of theinternal partially reflective surfaces;

FIG. 9 illustrates reflectance curves as a function of wavelength foranother coating that can be used in some of the internal partiallyreflective surfaces;

FIG. 10 illustrates reflectance curves achieved when using the coatingof FIG. 9 in combination with a complementary coating used on some ofthe internal partially reflective surfaces;

FIG. 11 is a schematic representation of a series of internal partiallyreflective surfaces having two complementary coatings arranged on eachof the internal partially reflective surfaces in alternating order;

FIGS. 12A and 12B are schematic side and front view representation of anoptical device having two optical waveguides, each having a set ofpartially reflective internal surfaces that can have complementarycoatings, for performing optical aperture expansion in two dimensions;

FIG. 13 is a schematic representation of another optical device havingtwo optical waveguides, each having a set of partially reflectiveinternal surfaces that can have complementary coatings, for performingoptical aperture expansion in two dimensions

FIG. 14 is a schematic representation of an LOE having a series ofinternal partially reflective surfaces, and illustrating the progressionof image illumination through the LOE and undesired reflection from oneof the internal partially reflective surfaces;

FIG. 15A is a schematic representation of one of the internal partiallyreflective surfaces of FIG. 14 implemented with a patterned reflectivecoating similar to the patterned reflective coatings of FIGS. 4 and 5 ,illustrating the transmission and reflection of light incident to thefront side of internal partially reflective surface;

FIG. 15B is a schematic representation of the partially reflectivesurface of FIG. 15A, illustrating the transmission and reflection oflight incident to the back side of the internal partially reflectivesurface;

FIG. 16A is a schematic representation of an internal partiallyreflective surface, similar to the internal partially reflective surfaceof FIGS. 15A and 15B, constructed and operative according to theteachings of an aspect of the present invention, having an amount ofreflection suppressing material deployed between the reflective portionsof the patterned reflective coating and the front side of the internalpartially reflective surface, and illustrating the transmission andreflection of light incident to the front side of internal partiallyreflective surface;

FIG. 16B is a schematic representation of the partially reflectivesurface of FIG. 16A, illustrating the transmission of light incident toone region on the back side of the internal partially reflective surfaceand the suppression of light incident to another region on the back sideof the internal partially reflective surface by the reflectionsuppressing material;

FIG. 17 is a schematic representation of a section of a light-guideoptical element (LOE) showing an internal partially reflective surfaceand a blemish in the form of an indentation formed at an interfaceregion between the internal partially reflective surface and an externalface of the LOE;

FIG. 18 is a schematic representation corresponding to FIG. 17 ,illustrating the progression of image illumination through the LOE andthe scattering effects on the image illumination imparted by theindentation;

FIG. 19 is a schematic representation of a section of a light-guideoptical element (LOE) constructed and operative according to theteachings of an aspect of the present invention, similar to the LOE ofFIGS. 17 and 18 , but having an amount of light absorbing materialapplied at the indentation, and illustrating the absorption of imageillumination by the light absorbing material;

FIG. 20 is a side view similar to FIG. 12A, but showing a blemish in theform of a chipped corner or edge of the one of the optical waveguides;and

FIG. 21 is a side view corresponding to FIG. 20 , showing an amount oflight absorbing material applied at the chipped corner or edge,according to the teachings of an aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention provide various light-guide opticalelements with internal partial reflectors, including light-guide opticalelements in which the internal partial reflectors have coatings appliedaccording to a complementary coating scheme, and light-guide opticalelements having reflection suppressing material applied to one or moreregions of an external surface or surfaces of the light-guide opticalelement.

The principles and operation of the various light-guide optical elementsaccording to present invention may be better understood with referenceto the drawings accompanying the description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways. Initially, throughout this document, references are madeto directions such as, for example, front and back, upper and lower,left and right, and the like. These directional references are exemplaryonly to illustrate the invention and embodiments thereof.

Referring now to the drawings, FIG. 1 illustrates an optical device inthe form of a light-guide optical element (LOE), generally designated10, constructed and operative according to a non-limiting embodiment ofthe present invention. The LOE 10 is formed as a light-transmittingsubstrate, constructed from a transparent material (such as glass), thathas a pair of parallel faces (also referred to as “major externalsurfaces” or “surfaces”) 12, 14, and a plurality of planar partiallyreflective surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c deployed withinthe substrate at an oblique angle to the parallel faces 12, 14. In thenon-limiting illustrated embodiment, the LOE 10 forms a slab-typewaveguide, i.e., where the other two dimensions of the LOE 10 are atleast an order of magnitude greater than the distance between theparallel faces 12, 14. The partially reflective surfaces (referred tohereinafter interchangeably as “internal surfaces”, “internal partialreflectors”, “partial reflectors” or “facets”) 16 a, 16 b, 16 c, 18 a,18 b, 18 c are subdivided into two sets of internals surfaces, namely afirst set 16 having the internal surfaces 16 a, 16 b, 16 c, and a secondset 18 having the internal surfaces 18 a, 18 b, 18 c. For simplicity ofpresentation each of the sets 16, 18 is illustrated here as having threeinternal surfaces, however it should be understood that either or bothof the sets could have any suitable number of internal surfaces.

In certain preferred but non-limiting embodiments, the internal surfacesof the two sets 16, 18 are interleaved, such that one or more of theinternal surfaces 16 a, 16 b, 16 c is positioned between a pair ofadjacent internal surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c, and viceversa.

Preferably, the internal surfaces alternate between the internalsurfaces of the two sets 16, 18, such that for each pair of adjacentinternal surfaces 16 a, 16 b, 16 c there is a single one of the internalsurfaces 18 a, 18 b, 18 c, and vice versa. This alternatingconfiguration is illustrated in FIG. 1 .

A projected image 20, represented here schematically by a beam ofillumination 20 including sample light rays 20A and 20B, is coupled intothe LOE 10 (i.e., into the substrate) by an optical coupling-inconfiguration 22, represented schematically as a coupling-in reflector.Other suitable coupling-in configurations for coupling imageillumination into the LOE 10, such as by use of a suitably angledcoupling prism or a diffractive optical element, are well-known in theart. The image illumination 20 is guided within the LOE 10 by repeatedinternal reflection at the parallel faces 12, 14 (i.e., the imageillumination 20 is trapped by internal reflection within the LOEsubstrate). In certain preferred but non-limiting implementations, thepropagation through the LOE 10 by internal reflection is in the form oftotal internal reflection (TIR), whereby incidence of the propagatingimage illumination 20 at the parallel faces 12, 14 at angles greaterthan a critical angle causes reflection of the illumination at theparallel faces 12, 14. In other non-limiting implementations, thepropagation through the LOE 10 by internal reflection is effectuated bya reflective coating (e.g., an angularly selective reflective coating)applied to the parallel faces 12, 14.

The image illumination 20 propagates through the LOE 10 until reachingthe series of internal surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c,where part of the image intensity is reflected out of the LOE 10 aslight rays 24A, 24B. In certain embodiments, such as the embodimentillustrated in FIG. 1 , the internal surfaces 16 a, 16 b, 16 c, 18 a, 18b, 18 c reflect the image illumination as reflected light rays 24A, 24Bso as to coupled part of the image intensity out of the LOE 10 towardthe eye of an observer. As will be discussed, in other embodiments theinternal surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c reflect the imageillumination as reflected light rays 24A, 24B so as to be coupled intoanother LOE for guiding between parallel faces of the other LOE and forcoupling out toward the observer's eye by a set of internal surfacesdeployed within the other LOE.

The image illumination 20 typically includes multiple components ofillumination, including, for example, different polarization componentsand different color (i.e., spectral) components. The internal surfaces16 a, 16 b, 16 c, 18 a, 18 b, 18 c are preferably formed fromtransparent plates or slabs having coatings applied to at least part ofthe sides or surfaces of the plates or slabs. The coatings are designedwith reflective characteristics such that the coatings are at leastpartially reflective to incident light having particularly correspondingcharacteristics in order to generate a desired reflectivity pattern forthe components of the illumination, the details of which will bedescribed in detail below. In general, at least part of the internalsurfaces 16 a, 16 b, 16 c have a coating with a reflectivitycharacteristic such that certain components of the image illuminationare reflected by the internal surfaces 16 a, 16 b, 16 c. At least partof the internal surfaces 18 a, 18 b, 18 c also have a coating with areflectivity characteristic that is complementary to the reflectioncharacteristic of the internal surfaces 16 a, 16 b, 16 c, such thatcomponents of the image illumination that are not sufficiently reflectedby the internal surfaces 16 a, 16 b, 16 c are suitably and sufficientlyreflected by the internal surfaces 18 a, 18 b, 18 c.

Before explaining the details of the design of the reflectors 16 a, 16b, 16 c, 18 a, 18 b, 18 c in further detail, it is noted that theprojected image illumination 20 is a collimated image, i.e., where eachpixel is represented by a beam of parallel rays at a correspondingangle, equivalent to light from a distant scene far from the observer(the collimated image may be referred to as being “collimated toinfinity”). Although the image 20 is represented here simplistically asa single ray corresponding to a single point of the image, typically thecentroid of the image, it is noted that the image in fact includes arange of angles to each side of the central ray, which are coupled intothe substrate with a corresponding range of angles, and are similarlycoupled out of the substrate at corresponding angles thereby creating afield of view corresponding to parts of the image arriving in directionsto the eye of the observer.

Each internal surface has opposing ends that define where the internalsurface respectively starts and stops. These opposing ends are referredto as a “starting end” and a “stopping end”. Looking at the internalsurfaces 16 a and 18 a, for example, it can be seen that the internalsurface 16 a has a starting end 17 a-1 and a stopping end 17 a-2, andthe internal surface 18 a has a starting end 19 a-1 and a stopping end19 a-2. The internal surfaces 16 a, 16 b, 16 c are preferably deployedwithin the LOE 10 such that each of the internal surfaces 16 b, 16 cstarts where the previous internal surfaces 16 a, 16 b ends in aprojection plane of the internal surfaces. In other words, the startingend 17 b-1 of the internal surface 16 b is aligned with the stopping end17 a-2 of the internal surface 16 a, and the starting end 17 c-1 of theinternal surface 16 c is aligned with the stopping end 17 b-2 of theinternal surface 16 b. In such a deployment, the facets 16 a, 16 b, 16 cappear as continuous and non-overlapping in the projection plane, whichin the non-limiting implementation illustrated in FIG. 1 is a plane thatis parallel to the planes of the surfaces 12, 14. This deploymentensures that there are no gaps between adjacent internal surfaces 16 a,16 b, 16 c in the primary light propagation direction through the LOE 10(arbitrarily illustrated as being from left to right along thehorizontal axis in FIG. 1 ), thereby preserving continuous apertureexpansion (i.e., aperture multiplication) for the components of lightreflected by the first set 16. Similarly, the internal surfaces 18 a, 18b, 18 c are preferably deployed within the LOE 10 such that each of theinternal surfaces 18 b, 18 c starts where the previous internal surfaces18 a, 18 b ends, thereby preserving continuous aperture expansion forthe components of light reflected by the second set 18. In other words,the starting end 19 b-1 of the internal surface 18 b is aligned with thestopping end 19 a-2 of the internal surface 18 a, and the starting end19 c-1 of the internal surface 18 c is aligned with the stopping end 19b-2 of the internal surface 18 b.

In embodiments in which the internal surfaces of the two sets 16, 18 areinterleaved, it is preferable that the two sets 16, 18 are also inoverlapping relation whereby at least some of the internal surfaces ofthe first set 16 overlap with some of the internal surfaces of thesecond set 18, and vice versa. In certain cases, the overlappingrelation is such that there is at least one internal surface of one ofthe sets 16, 18 that has its starting end located at a position in theprojection plane that is between the starting and stopping ends of asingle internal surface of the other of the sets 16, 18, and such thatthe stopping end of the internal surface of the one of the sets 16, 18is located at a position in the projection plane that is between thestarting and stopping ends of another single internal surface of theother of the sets 16, 18.

FIG. 1 shows the two sets 16, 18 in an interleaved and overlappingconfiguration in which the starting end 19 a-1 of the internal surface18 a is located at a position in the projection plane that is betweenthe starting end 17 a-1 and the stopping end 17 a-2 of the internalsurface 16 a, the stopping end 19 a-2 of the internal surface 18 a islocated at a position in the projection plane that is between thestarting end 17 b-1 and the stopping end 17 b-2 of the internal surface16 b, the starting end 19 b-1 of the internal surface 18 b is located ata position in the projection plane that is between the starting end 17b-1 and the stopping end 17 b-2 of the internal surface 16 b, thestopping end 19 b-2 of the internal surface 18 b is located at aposition in the projection plane that is between the starting end 17 c-1and the stopping end 17 c-2 of the internal surface 16 b, and thestarting end 19 c-1 of the internal surface 18 c is located at aposition in the projection plane that is between the starting end 17 c-1and the stopping end 17 c-2. Likewise, the stopping end 17 a-2 of theinternal surface 16 a is located at a position in the projection planethat is between the starting end 19 a-1 and the stopping end 19 a-2 ofthe internal surface 18 a, the starting end 17 b-1 of the internalsurface 16 b is located at a position in the projection plane that isbetween the starting end 19 a-1 and the stopping end 19 a-2 of theinternal surface 18 a, the stopping end 17 b-2 of the internal surface16 b is located at a position in the projection plane that is betweenthe starting end 19 b-1 and the stopping end 19 b-2 of the internalsurface 18 b, the starting end 17 c-1 of the internal surface 16 c islocated at a position in the projection plane that is between thestarting end 19 b-1 and the stopping end 19 b-2 of the internal surface18 b, and the stopping end 17 c-2 of the internal surface 16 c islocated at a position in the projection plane that is between thestarting end 19 c-1 and the stopping end 19 c-2 of the internal surface18 c.

Preferably the overlapping configuration between the internal surfacesof the two sets 16, 18 is such that the starting/stopping end of aninternal surface of one of the sets 16, 18 is at the midpoint betweenthe starting and stopping ends of the internal surface of the other ofthe sets 16, 18. It should be noted that in certain instances“overlapping relation” may include configurations in which an internalsurface of the set 16 and an internal surface of the set 18 are entirelyoverlapping such that they are coplanar, whereby the starting andstopping ends of a facet of the set 16 are respectively coincident withthe starting and stopping ends of a facet of the set 18. Further detailsof optical waveguides that employ overlapping internal surface havingconventional coating architectures can be found in the applicant'scommonly owned U.S. Pat. No. 10,481,319, which is incorporated byreference in its entirety herein.

The following paragraphs describe the coating designs for the sets 16,18 of internal surfaces according to embodiments of the presentinvention. The internal surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c havecoatings with complementary reflectivity characteristics such thatcomponents of the image illumination that are not sufficiently reflectedby one of the internal surfaces 16 a, 16 b, 16 c are suitably andsufficiently reflected by the one of the internal surfaces 18 a, 18 b,18 c. In particular, and as will be described in detail below, theinternal surfaces 16 a, 16 b, 16 c have coatings configured to reflect aproportion of intensity for each illumination component in a subset ofthe components of the image illumination, and the internal surfaces 18a, 18 b, 18 c have coatings configured to reflect a proportion ofintensity for each illumination component in another subset of thecomponents of the image illumination, such that the coatings of the twosets 16, 18 of internal surfaces cooperate to reflect a combinedproportion of intensity of all illumination components in the twosubsets. The combined proportion of intensity cooperatively reflected bythe coatings of the two sets 16, 18 is greater than or equal to theproportion of intensity reflected individually by the coatings of thetwo sets 16, 18.

When the internal surfaces are interleaved according to the alternatingconfiguration as illustrated in FIG.1, the complementary coatings ofpairs of adjacent internal surfaces from two different sets enable theinternal surfaces from the two sets to cooperate to reflect all of thecomponents of the image illumination across portions of the projectionplane of the internal surfaces so as to preserve continuous apertureexpansion.

As part of a first non-limiting example, image illumination 20 thatincludes different spectral components of illumination, for examplespectral components corresponding to red light, green light, and bluelight, is considered. In this example, the internal surfaces 16 a, 16 b,16 c may include a first coating that is configured to reflect red light(i.e., light having wavelengths near 638 nm) with high efficiency and topartially reflect green light (i.e., light having wavelengths near 532nm) with moderate efficiency, but is configured to partially reflectblue light (i.e., light having wavelengths near 456 nm) with lowefficiency. In order to compensate for the moderate reflectionefficiency of green light and the low reflection efficiency of bluelight imparted by the internal surfaces 16 a, 16 b, 16 c, the internalsurfaces 18 a, 18 b, 18 c can include a second coating that isconfigured to reflect blue light with high efficiency (on par with theefficiency imparted by the internal surfaces 16 a, 16 b, 16 c on redlight) and to partially reflect green light with moderate efficiency (onpar with the efficiency imparted by the internal surfaces 16 a, 16 b, 16c on green light). The coating of the internal surfaces 18 a, 18 b, 18 cmay also partially reflect red light with low efficiency. As a result,the light rays 24A convey high efficiency red light, moderate efficiencygreen light, and low efficiency blue light, and the light rays 24Bconvey high efficiency blue light and moderate efficiency green light,such that the overall reflected image resultant from the reflection bythe two interleaved and overlapping sets 16, 18 has little to no colordifference across the three colors while preserving continuous apertureexpansion (due to the interleaving of the internal surfaces). Anyresidual color difference that cannot be eliminated by the coatings ofthe two sets 16, 18 can be compensated for by adjustment of coloredlight sources that are used for generating the collimated imageillumination 20.

In another non-limiting example, image illumination 20 that includes twoorthogonal linear polarization components, namely s-polarization andp-polarization components, is considered. Here, the two sets 16, 18 ofinternal surfaces include coatings that are selectively reflective tothe orthogonal polarizations in a complementary fashion, whereby theinternal surfaces of one of the sets 16 primarily reflect light that ispolarized in one of the polarization directions (e.g., p-polarization)with respect to the surface of the internal surfaces of the set 16, andthe internal surfaces of the other set 18 primarily reflect light thatis polarized in the orthogonal polarization direction (e.g.,s-polarization) with respect to the surface of the internal surfaces ofthe set 18.

One type of coating that can provide such polarization selectivereflectivity is a dielectric coating. FIG. 2 shows the reflectivitycharacteristics of such a dielectric coating for p-polarization ands-polarization across angle of incidence (AOI). As can be seen, at alower range of AOIs, for example AOIs in the range of 0-20 degrees(i.e., close to perpendicular the internal surfaces), both s andp-polarizations are reflected with approximately the same efficiency,i.e., the reflectance of s and p-polarizations is approximately the same(slightly above 25%). As the AOI increases over a given range, thereflectance of the two polarizations deviates. Specifically, at a higherrange of AOIs, for example AOIs in the range of 20-55 degrees, thereflectance for p-polarization is reduced relative to the reflectancefor s-polarization. For example, at AOI of approximately 40 degrees, thereflectance for s-polarization is slightly above 50% (thereby operatingas an almost perfect partial reflector), whereas the reflectance forp-polarization is below 15%.

In order to generate an image having wide field of view for theobserver, different angles are reflected from different internalsurfaces. FIG. 3 shows the LOE 10 in which all of the internal surfaces16 a, 16 b, 16 c, 18 a, 18 b, 18 c include the dielectric coating havingthe reflectance characteristics described above with reference to FIG. 2. In this configuration, the image illumination that propagates throughthe LOE has both s-polarization and p-polarization components. By way ofillustration, some of image illumination that propagates through the LOE10 impinges on the internal surface 18 c at an AOI in the lower rangesuch that the dielectric coating reflects both polarizations withapproximately the same efficiency. As a result, the polarizationcomponents of the reflected light ray R_(18c) are of approximately equalintensity. However, some of the image illumination impinges on theinternal surfaces 18 a, 18 b, 16 b at AOIs in the higher range, suchthat the dielectric coating of the internal surfaces 18 a, 18 b, 16 bprimarily reflects the s-polarized light. As a result, thes-polarization component of each of the reflected light rays R_(18a),R_(18b), R_(16b) is the dominant component. In order to compensate forthe reduced p-polarization component at the particular AOI range, theinternal surfaces 18 a, 18 b are re-designed so as to reflect primarilyp-polarized light (or to reflect both polarizations with approximatelyequal efficiency).

According to certain embodiments, in order to achieve the desiredreflectivity for p-polarized light, the internal surfaces 18 a, 18 badditionally include an orientation sensitive polarization reflector (or“structural polarizer”) that transmits one incident polarization andreflects the orthogonal polarization according to the reflector'sinherent axis orientation. One non-limiting example of a structuralpolarizer is a birefringent dielectric coating or film commerciallyavailable from the 3M Company of Minnesota, USA. Another non-limitingexample of a structural polarizer is a wire-grid film, for examplecommercially available from Moxtek Inc. of Utah, USA. Yet anothernon-limiting example of a structural polarizer is a patterned partiallyreflective coating having a number of portions of reflective materialdeployed in a pattern on a thin film or transparent substrate.

With continued reference to FIGS. 1-3 , refer now to FIG. 4 , whichshows an illustration of a non-limiting example of a patternedreflective coating (also referred to as a “reflective pattern coating”)30 according to non-limiting embodiments of the present invention. Thecoating 30 has reflective characteristics such that light that ispolarized in one polarization direction (e.g., s-polarized orp-polarized) is primarily/majority reflected by the coating 30, andlight that is polarized in the orthogonal polarization direction (e.g.,p-polarized or s-polarized) is primarily/majority transmitted by thecoating 30. Preferably, the reflected polarization exhibits more than90% reflection (referred to as “substantially completely reflective”),and most preferably over 95% reflection. Conversely, the transmittedpolarization preferably exhibits more than 90% transmission (referred toas “substantially completely transmissive”), and most preferably over95% transmission.

The coating 30 includes an amount 34 of reflective material (referred tohereinafter as “portions” 34) deployed in spaced relation and arrangedin a prescribed pattern on a planar base surface 32. The base surface 32is preferably, but not necessarily, transparent to light such that thespaces 35 on the base surface 32 that are formed between and around theportions 34 of reflective material are light-transparent. In certainembodiments, the planar base surface 32 is a thin-film or thin-substratethat can be bonded to a transparent plate to form the internal partiallyreflective surface. In other embodiments, the planar base surface 32 isitself the transparent plate from which the facet is formed, and theportions 34 of reflective material are deposited directly on thetransparent plate. In certain embodiments, the reflective material is adielectric material. In other sometimes more preferred embodiments, thereflective material is a metallic material, such as silver. Each portion34 of the reflective material has a shape that enables light in onepolarization direction to induce flow of electrical current. Therefore,light that is polarized in the polarization direction that inducescurrent flow sees the coating 30 as a reflector when incident to thecoating 30, whereas light that is polarized in the orthogonalpolarization direction sees the coating 30 as light-transmissive whenincident to the coating 30.

In the non-limiting example illustrated in FIG. 4 , each of the portions34 is identical in size and each has a generally circular shape in theplane of the base surface 32 (i.e., in the plane of the internalsurface). Here, the portions 34 are effectively circularly symmetric (inthe plane of the base surface 32) dots of reflective material depositedon the base surface 32 in the arranged pattern. In this configuration,the portions 34 are arranged in a prescribed pattern so as to beuniformly spaced such that the distance between the centers of each pairof adjacent dots is constant across the entire coating 30.

FIG. 5 shows another non-limiting example of the coating 30 in whichportions 36 of reflective material having non-circular symmetry in theplane of the base surface 32 are deployed on the base surface 32 in aprescribed pattern. Here, the portions 36 have a generally elliptical oroblong shape (two orthogonal axes of symmetry) in the plane of the basesurface 32 (i.e., in the plane of the internal surface). Theorientations of the portions 36 in the plane of the base surface 32determine the dominant reflective polarization. For example, in theconfiguration of the portions 36 illustrate in FIG. 5 , the dominantreflected polarization may be p-polarization, whereas rotating theportions 36 by 90-degrees in the plane of the base surface 32 may switchthe dominant reflected polarization to s-polarization. Other shapes ofthe reflective material besides circular and oblong shapes arecontemplated herein, for example, the portions of reflective materialmay be deployed in a pattern of lines on the base surface 32.

By employing internal surfaces 18 a, 18 b that have the coating 30, theinternal surfaces 18 a, 18 b are able to reflect the subset of theillumination components (in this case the p-polarization components)that is not fully reflected by the internal surface 16 b. In otherwords, for a given AOI in the higher AOI range, the internal surface 16a reflects a first subset of components of the image illumination (inthe form of the s-polarization components) with high reflectance andreflects a second subset of components of the image illumination (in theform of the p-polarization components) with low reflectance. For thesame given AOI, the internal surfaces 18 a, 18 b reflect the lowreflectance components, i.e., the second subset of components of theimage illumination (in this case the p-polarization components) withhigh reflectance, so as to compensate for the low reflectance impartedby the internal surface 16 b. As a result, the internal surfaces 18 a,16 b, 18 b cooperate to reflects both polarization components (i.e., thecomponents from both subsets) to preserve continuity of aperturemultiplication. The two subsets of components of image illumination arecomplementary, meaning that the union of the components from the twosubsets accounts for all of the components of the propagating imageillumination. In this particular example, the s and p-polarizationcomponents are complementary since they make up the polarizationcomponents of the image illumination.

In certain embodiments, two different coatings may be implemented on thesame internal surface plane using a single coating. For example, adielectric coating can be deployed in the spaces between the portions34. As a result, the portions 34 or 36 can be implemented as one type ofdielectric coating or metallic coating, and the spaces 35 on the basesurface 32 that are formed between and around the portions 34 or 36 canbe implemented as another type of dielectric coating. FIG. 6schematically illustrates an example of such a coating 31, in whichportions 38 of a second reflective material are deposited in aprescribed pattern in the spaces 35 on the base surface 32 formedbetween and around the portions 34. In the non-limiting exampleillustrated in FIG. 6 , each of the portions 34 is generally circular inshape, whereas each of the portions 38 is generally elliptical in shape.

As discussed, the coating designs of the embodiments of the presentinvention are equally applicable to situations in which the imageillumination includes different visible color components. In suchsituations, some of the principles of the patterned reflector coatingsdescribed above with reference to FIGS. 4-6 can be used to address colornon-uniformity issues. For example, the internal surfaces 16 a, 16 b, 16c can include a coating that partially reflects a first subset of thethree colors at a suitable reflection efficiency, and the internalsurfaces 18 a, 18 b, 18 c can include a coating that partially reflectsa second subset of the three colors at a suitable efficiency, where thesecond subset of the colors includes colors that are not suitablyreflected by the internal surfaces 16 a, 16 b, 16 c. In general, thesubsets of color components of image illumination are complementary,meaning that the union of the components from the subsets accounts forall of the color components of the propagating image illumination. Thefollowing paragraphs describe various examples of designs of thecoatings of the internal surfaces of the two sets 16, 18 for preservingcolor uniformity.

By way of introduction, it would be preferable to arrange the portions34, 36 of reflective material in a pattern that is relatively small sothat the observer will perceive a uniform image. In particular, it wouldbe preferable to deploy the portions 34, 36 of reflective material in ageometric arrangement in accordance with the size of the pupil of theeye of the observer, for example as a circle having a diameter ofapproximately 2 mm (the pupil of the human eye typically has a diameterin the range of 2-4 mm in bright lighting conditions). However, portionsof reflective material having small size and arranged in small patternstend to diffract incident light to large angles, thereby reducing imageresolution. Therefore, in non-limiting implementations of the presentinvention, the internal surfaces of the two sets 16, 18 are implementedusing the coatings having reflective patterns (described above withreference to FIGS. 4-6 ) in combination with dielectric coatings.

In one non-limiting example, the internal surfaces 16 a, 16 b, 16 c areimplemented using a dielectric coating so as to be at least partiallyreflective to red, green and blue light, and the internal surfaces 18 a,18 b, 18 c are implemented using a patterned coating 30 in which thereflective material of the coating 30 is a metallic material (e.g.,silver). The dielectric coating of the internal surfaces 16 a, 16 b, 16c has reflection characteristics according to the graph illustrated inFIG. 7 . Here, the dielectric coating of the internal surfaces 16 a, 16b, 16 c reflects a first subset of components of the image illumination,in the form of green light (i.e., light having wavelengths near 532 nm),with reasonably high efficiency (approximately 10% reflectance), butreflects a second subset of components of the image illumination, in theform of red light and blue light (i.e., light having wavelengths near638 nm and 456 nm, respectively), with lower efficiency than the greenlight reflection (approximately 4% reflectance). The coating 30 of theinternal surfaces 18 a, 18 b, 18 c has reflection characteristics so asto be reflective for both subsets of components with enough efficiencyin order to compensate for the low reflectance of the second subset ofcomponents. The overall reflectance imparted by the combination of thedielectric coating of the internal surfaces 16 a, 16 b, 16 c and thecoating 30 of the internal surfaces 18 a, 18 b, 18 c is illustrated inFIG. 8 . As can be inferred, the coating 30 reflects the second subsetof components of the image illumination (i.e., red light and blue light)with reflectance of at least approximately 6%, which is a higherefficiency than that imparted on the second subset of components by thedielectric coating of the internal surfaces 16 a, 16 b, 16 c. Thecoating 30 also reflects the first subset of components of the imageillumination (i.e., green light) with reflectance of approximately 4%reflectance. The two subsets of color components are complementary inthat the union of the two subsets (first subset having high efficiencygreen light, second subset having high efficiency red and blue light)accounts for all three of the color components of the imageillumination. As a result, the overall reflected image has a reducedcolor difference, albeit while having a higher resolution of the greencolor components than the red and blue color components. The human eye,however, is most sensitive to the resolution of green light componentsof an image, and therefore an overall image having higher resolution ofgreen color components would likely be perceived by the observer as nothaving any noticeable resolution degradation.

In an alternative configuration, the coating 30 can be implemented usinga reflective material that has higher reflectance for red light and bluelight than for green light (i.e., the coating 30 reflects mostly redlight and blue light). As a result, the overall reflected image wouldhave little to no noticeable color difference.

In another non-limiting example, the internal surfaces 16 a, 16 b, 16 care implemented using a dielectric coating that has reflectioncharacteristics according to the graph illustrated in FIG. 9 . Here, thedielectric coating of the internal surfaces 16 a, 16 b, 16 c reflects afirst subset of components of the image illumination, in the form ofgreen light and red light, with high efficiency (approximately 15%reflectance), but reflects a second subset of components of the imageillumination, in the form of blue light, with lower efficiency than thegreen light and red light reflection (approximately 10% reflectance). Inorder to compensate for the low reflectance of the second subset ofcomponents, a particular implementation of the coating 30 is used forthe internal surfaces 18 a, 18 b, 18 c. In this implementation, theportions of the reflective material (implemented as dielectric materialor metallic material) are small (preferably in accordance with the humanpupil size discussed above), and have reflection characteristics suchthat only blue light is reflected by the coating 30. The overallreflectance imparted by the combination of the dielectric coating of theinternal surfaces 16 a, 16 b, 16 c and the coating 30 of the internalsurfaces 18 a, 18 b, 18 c is illustrated in FIG. 10 , whereby theoverall reflectance is approximately constant at approximately 15%across the visible light spectrum. The result is a white balanced imagewithout diffraction (blue light tends to be diffracted much less thangreen and red light).

FIG. 11 shows another implementation of using two coating schemes topreserve color uniformity according to a non-limiting example. Here, theinternal surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c have two sets ofcoatings on each reflector arranged in an alternating configuration,where there is a lateral change in the coating of each internal surface.In the non-limiting illustrated example, each internal surface has twonon-overlapping portions, namely a first portion and a second portion.The first portions 40 a, 40 b, 40 c of the internal surfaces 16 a, 16 b,16 c have a first coating 33, for example a dielectric coating havingreflective characteristics according to FIG. 7 or FIG. 9 , and thesecond portions 42 a, 42 b, 42 c of the internal surfaces 16 a, 16 b, 16c have a second coating 37, for example the coating 30. The firstportions 44 a, 44 b, 44 c of the internal surfaces 18 a, 18 b, 18 c havethe second coating 37, and the second portions 46 a, 46 b, 46 c of theinternal surfaces 18 a, 18 b, 18 c have the first coating 33.

In the non-limiting example illustrated in FIG. 11 , the coatings 33, 37are deployed on alternating portions of successive internal surfaces,such that the coatings on each pair of adjacent internal surfaces (e.g.,internal surfaces 16 a, 18 a, internal surfaces 18 a, 16 b, internalsurfaces 16 b, 18 b, etc.) cooperate to reflect all of the components ofsubsets of the image illumination with reasonable efficiency so as topreserve color uniformity. In this particular configuration, the twosets of internal surfaces can be thought of as being effectivelycoplanar, whereby each internal surface has both coatings 33, 37. It isnoted that although FIG. 11 shows each of the two portions of each ofthe internal surfaces constitutes approximately half of the internalsurface plane, other configurations are possible, so long as theportions of the internal surfaces on which the coatings are deployedalternate between successive internal surfaces.

Although the embodiments for preserving color uniformity have beendescribed within the context of the internal surfaces 16 a, 16 b, 16 chaving dielectric coatings, and the internal surfaces 18 a, 18 b, 18 chaving the coatings implemented according to the coating 30, and inwhich the internal surfaces 16 a, 16 b, 16 c and 18 a, 18 b, 18 c areinterleaved, other embodiments are possible in which both types ofcoatings are implemented on a single internal surface, for example asdiscussed above with reference to FIGS. 4-6 . For example, each of theinternal surfaces 16 a, 16 b, 16 c, 18 a, 18 b, 18 c may include twocoatings: 1) a first coating, for example the coating 30, and 2) asecond coating, for example a dielectric deployed in the spaces formedbetween the portions 34 of the coating 30. The second coating may havethe reflection characteristics according to FIG. 7 or FIG. 9 , whereby afirst subset of the components of the image illumination is reflected bythe second coating with higher efficiency than a second subset ofcomponents of the image illumination. The first coating may then havereflection characteristics which compensate for the low reflectanceimparted on the second subset by the second coating, such that eachindividual internal surface achieves an overall reflectance that isapproximately uniform across the three colors, for example asillustrated in FIGS. 8 and FIG. 10 . In such a configuration, it is notnecessary for the two sets 16, 18 to be interleaved. Instead, since theinternal surfaces of both of the sets 16, 18 are identically coated, thetwo sets 16, 18 are one in the same, and are preferably deployed suchthat each internal surface starts where the previous internal surfaceends.

In certain embodiments, the patterned reflective coating 30 of theinternal surfaces may be configured such that the number of portions 34,36 and/or or the size of the portions 34, 36 on the internal surfacesvaries from facet to facet in order to provide uniform intensity acrossthe entire field of view. For example, the internal surfaces 16 a, 16 b,16 c may be implemented using dielectric coatings (as discussed above),and the internal surfaces 18 a, 18 b, 18 c may be implemented using thepatterned reflective coating 30. As light propagates through the LOE,the intensity of the light that impinges on each successive facet isless than the intensity of the light that impinges on the precedingfacet. This is due to the fact that a proportion of the intensity oflight that impinges on a particular facet is reflected out of the LOE bythat particular facet. In order to compensate for the decrease in lightintensity in the light propagation direction, the reflectance impartedby each facet should generally increase compared to the reflectanceimparted by the preceding facet. This can be effectuated by increasingthe density of the reflective material on the coating 30 on the internalsurfaces of the second set 18 with respect to the primary propagationdirection of light through the LOE by, for example, increasing thenumber of portions 34, 36 and/or or the size of the portions 34, 36. Forexample, the coating 30 of the internal surface 18 a can be implementedwith a first number of portions 34, 36 and/or or a first size of theportions 34, 36, the coating 30 of the internal surface 18 b can beimplemented with a second number of portions 34, 36 and/or or a secondsize of the portions 34, 36, and the coating 30 of the internal surface18 c can be implemented with a third number of portions 34, 36 and/or ora third size of the portions 34, 36. The first number of portions isless than the second number of portions, which is less than the thirdnumber of portions, and the first size of the portions is less than thesecond size of the portions, which is less than the third size of theportions.

Although some of the embodiments described thus far have pertained totwo sets of internal partial reflectors that have complementarycoatings, other embodiments are possible in which there are more thantwo sets of partial reflectors having complementary coatings. As asimple example, a third set of internal surfaces can be deployedparallel to, and interleaved with, the internal surfaces of the othertwo sets 16, 18. Each set of internal surfaces can include a coatingthat is configured to reflect a particular subset of components of theimage illumination. For example, the coating of the internal surfaces ofthe first set can be configured to primarily reflect red light, thecoating of the internal surfaces of the second set can be configured toprimarily reflect green light, and the coating of the internal surfacesof the third set can be configured to primarily reflect blue light. As aresult, a given group of three (preferably consecutive) internalsurfaces (the group having one internal surface from each of the threesets) is able cooperate to reflect all three components of imageillumination.

The coating and facet deployment methodologies discussed above have beendescribed within the non-limiting example context of image illuminationhaving either different spectral components or different polarizationcomponents. However, it should be appreciated that image illuminationoften has both spectral and polarization components (e.g., linearlypolarized red, green, and blue light). For image illumination thatimpinges on the facets at a higher range of AOIs, e.g., 20-50 degrees,the coatings of the sets of facets can be designed to satisfy bothspectral and polarization requirements to achieve transmissionequalization across a wide field of view.

Although the coating designs and the deployment of the internal surfaceshave thus far been described within the context of an LOE in which lightis guided through the LOE in one dimension and is coupled-out (as“unguided” light) by the internal surfaces (facets) so as to performaperture expansion in one dimension (performing what is referred toherein as “guided-to-unguided” image propagation), the coating designand facet deployment described herein according to embodiments of thepresent invention are equally applicable to optical devices having atleast two optical waveguides that cooperate to guide light intwo-dimensions in order to perform aperture expansion in two dimensions.These types of optical devices perform what is referred to herein as“guided-to-guided” image propagation, whereby image illumination isguided through a first optical waveguide (in one or two dimensions) andis reflected by a set of facets deployed in the first optical waveguideso as to be coupled into a second optical waveguide. The imageillumination is then guided through the second optical waveguide (in onedimension) and is reflected by a set of facets deployed in the secondoptical waveguide so as to couple the image illumination out of thesecond optical waveguide for viewing by an observer. The followingparagraphs provide examples of optical devices that performguided-to-guided image propagation.

FIGS. 12A and 12B show schematic side and front views, respectively, ofan optical device that performs guided-to-guided image propagation byway of two optical waveguides 50, 60 that are optically coupledtogether. The optical waveguide 50 has a direction of elongationillustrated arbitrarily as corresponding to the “x-axis”, and includestwo pairs of parallel faces (i.e., major external surfaces) 52 a, 52 b,54 a, 54 b forming a rectangular cross-section. A plurality of mutuallyparallel internal partially reflecting surfaces (i.e., facets) 58 atleast partially traverse the optical waveguide 50 at an oblique angle tothe direction of elongation. The optical waveguide 60, optically coupledto the optical waveguide 50, has a pair of parallel faces 62 a, 62 bforming a slab-type waveguide. Here too, a plurality of mutuallyparallel internal partially reflecting surfaces (i.e., facets) 64 atleast partially traverse the optical waveguide 60 at an oblique angle tothe parallel faces 62 a, 62 b. The planes containing the facets 58 areoblique to the planes containing the facets 64.

The optical coupling between the optical waveguides 50, 60, and thedeployment and configuration of partially reflecting surfaces 58, 64 aresuch that, when an image is coupled into the optical waveguide 50 withan initial direction of propagation at a coupling angle oblique to boththe first and second pairs of parallel faces 52 a, 52 b, 54 a, 54 b, theimage advances by four-fold internal reflection along the opticalwaveguide 50 (i.e., in two dimensions), with a proportion of intensityof the image reflected at the partially reflecting surfaces 58 so as tobe coupled out of the optical waveguide 50 and into the opticalwaveguide 60, and then propagates through two-fold internal reflectionwithin the optical waveguide 60 (i.e., in one dimension, similar to asin the LOE 10), with a proportion of intensity of the image reflected atthe partially reflecting surfaces 64 so as to be coupled out of theoptical waveguide 60 as a visible image seen by the eye of an observer.As a result of this construction, the light that propagates through theoptical waveguide 50 is guided (in two dimensions by the opticalwaveguide 50), and the light that is reflected by the partiallyreflecting surfaces 58 is also guided (in one dimension by the opticalwaveguide 60).

The coating design principles and/or the facet interleaving principlesaccording to the embodiments of the present invention can be applied toeither or both of the sets of internal partially reflecting surfaces 58,64. Further details of such an optical device that employs two opticalwaveguides 50, 60 can be found in the applicant's commonly owned U.S.Pat. No. 10,133,070, which is incorporated by reference in its entiretyherein.

FIG. 13 shows a schematic view of an optical device that performsguided-to-guided image propagation by way of two slab-type opticalwaveguides 70, 80 that are optically coupled together. The opticalwaveguide 70 has two pairs of parallel faces 72 a, 72 b, 74 a, 74 bforming a slab-type waveguide (in the figure the faces 72 a, 72 b are atthe front and back, respectively, of the optical waveguide 70, and thefaces 74 a, 74 b are at the left and right, respectively, of the opticalwaveguide 70). A plurality of mutually parallel internal partiallyreflecting surfaces (i.e., facets) 76 at least partially traverse theoptical waveguide 70 at an oblique angle to the parallel faces 72 a, 72b, 74 a, 74 b. The optical waveguide 80 has two pairs of parallel faces82 a, 82 b, 84 a, 84 b forming a slab-type waveguide (in the figure thefaces 82 a, 82 b are at the front and back, respectively, of the opticalwaveguide 80, and the faces 84 a, 84 b are at the left and right,respectively, of the optical waveguide 80). A plurality of mutuallyparallel internal partially reflecting surfaces (i.e., facets) 86 atleast partially traverse the optical waveguide 80 at an oblique angle tothe parallel faces 82 a, 82 b, 84 a, 84 b. In addition, the planescontaining the facets 76 are oblique or perpendicular to the planescontaining the facets 86.

In the illustrated non-limiting implementation, the optical waveguides70, 80 are optically coupled together in a configuration in which theoptical waveguide 70 is stacked on top of the optical waveguide 80.Note, however, the optical waveguides 70, 80 can be stacked front toback (e.g., with the faces 72 b, 82 a in facing relation to each other).The optical coupling between the optical waveguides 70, 80, and thedeployment and configuration of partially reflecting surfaces 76, 86 aresuch that, when an image is coupled into the optical waveguide 70, theimage propagates through two-fold internal reflection within the opticalwaveguide 70 between the faces 72 a, 72 b in a first guided direction,with a proportion of intensity of the image reflected at the partiallyreflecting surfaces 76 so as to be coupled out of the optical waveguide70 and into the optical waveguide 80, and then propagates throughtwo-fold internal reflection within the optical waveguide 80 between thefaces 82 a, 82 b in a second guided direction (oblique to the firstguided direction), with a proportion of intensity of the image reflectedat the partially reflecting surfaces 86 so as to be coupled out of theoptical waveguide 80 as a visible image seen by the eye of an observer.

The coating design principles and/or the facet interleaving principlesaccording to the embodiments of the present invention can be applied toeither or both of the sets of internal partially reflecting surfaces 76,86. Further details of such an optical device that employs two opticalwaveguides 70, 80 can be found in the applicant's commonly owned U.S.Pat. No. 10,551,544, which is incorporated by reference in its entiretyherein.

While the use of the reflective pattern coatings disclosed herein hasthe benefit of preserving color uniformity and intensity uniformity, theuse of the reflective pattern coatings may cause undesired reflectionsfrom the internal surfaces, which can lead to ghost images. The generalconcept of undesired reflections from the internal surfaces is describedwith reference to FIG. 14 . Here, LOE 100 has three mutually parallelpartially reflective internal surfaces 106 a, 106 b, 106 c deployedoblique to a pair of parallel faces (major external surfaces) 102, 104.The thickness of the internal surfaces 106 a, 106 b, 106 c isexaggerated in FIG. 14 for clarity of illustrating front sides 108 a,108 b, 108 c and back sides 110 a, 110 b, 110 c of the internal surfaces106 a, 106 b, 106 c. The front and back sides of an internal surface aregenerally opposing sides, where the front side is the side of theinternal surface that is coated with the coatings (described withreference to FIGS. 1-11 ) having the reflective characteristics thatenable reflection of the propagating image illumination according to thedesired reflectivity pattern.

Image illumination 108, schematically represented by light ray 108, iscoupled into the LOE 100 by the coupling-in reflector 110 (or any othersuitable optical coupling-in configuration, e.g., coupling prism, etc.).The image illumination 108 propagates through the LOE 100 by repeatedinternal reflection at the faces 102, 104 (either by total internalreflection or due to an angularly selective reflective coating appliedat the faces), until reaching the series of internal surfaces 106 a, 106b, 106 c, where part of the image intensity is reflected, at the frontsides 108 a, 108 b, 108 c of the internal surfaces 106 a, 106 b, 106 c,out of the LOE 100 as light rays 116 a-116 d. Looking at the propagatingimage illumination 118 schematically represented by the light ray 118,it can be seen that part of the intensity of the light ray 118 istransmitted by the internal surface 106 a (as light ray 120) after whichthe light ray 120 is reflected at the face 102 and then a proportion ofthe intensity is reflected at the front side 108 a of the internalsurface 106 a so as to be reflected out of the LOE 100 as light ray 116b (the remaining intensity is transmitted by the internal surface 106 a,such that the light continues propagating through the LOE 100). However,part of the intensity of the light ray 118 undergoes an undesiredreflection at the back side 110 a of the internal surface 106 a,resulting in reflected ray 122. The reflected ray 122 can, in certaincircumstances, undergo internal reflection at the faces 102, 104,exemplified by the reflection at the face 102, so as to generatereflected ray 124. The reflected ray 124 is reflected at the front side108 b of the internal surface 106 b so as to be reflected out of the LOE100 as ghost light ray 126.

FIGS. 15A and 15B show how the reflective pattern coating 30 enablesboth desired reflections at the front side of an internal surface andundesired reflections at the back side of the internal surface. It isnoted that FIGS. 15A and 15B are not drawn to scale, and some of thedimensions of the internal surface and the components of the reflectivepattern coating 30 are exaggerated for clarity of illustration.

Looking first at FIG. 15A, there is shown how an arbitrary internalsurface 130 (which can be for example one of the internal surfaces ofthe set 18) handles propagating image illumination 140 that impinges onthe front side 132 of the internal surface 130. The internal surface 130has the reflective pattern coating 30 deposited on the front side 132 ofthe internal surface 130. In particular, the planar base surface 32 isdeposited on the front side 132 such that the portions 34 are arrangedin the desired pattern on the front side 132. Alternatively, theportions 34 can be deposited directly on the front side 132 in thearranged pattern without the planar base surface 32. Propagating imageillumination 140, represented schematically by light rays 140A and 140B,impinges on different regions of the front side 132 of the internalsurface 130. In this case, the propagating image illumination 140 is theimage illumination that has undergone reflection at the lower face ofthe LOE (e.g., the face 102 in FIG. 14 or the face 12 in FIG. 1 ). Thepart of the propagating image illumination represented by the light ray140A impinges on a region of the internal surface 130 having thereflective material so as to be reflected (out of the LOE) by one of theportions 34 of reflective material as reflected light ray 142. The partof the of the propagating image illumination represented by the lightray 140B impinges on a region of the internal surface 130 having spaces35 between the portions 34 of reflective material, and is transmitted bythe internal surface 130 as light ray 142 (i.e., the light ray 140Bpasses through the internal surface 130 from the front side 132 to theback side 134 as light ray 142, due to the spaces 35 being transparent).This light ray 140B continues to propagate through the LOE, beingreflected at the faces of the LOE and/or reflected by subsequentinternal surfaces. As a result, part of the image illumination 140A isreflected out of the LOE by the internal surface 130, and part of theimage illumination 140B is transmitted by the internal surface 130.

Turning now to FIG. 15B, there is shown how the internal surface 130handles propagating image illumination 118, represented schematically bylight rays 118A and 118B, that impinges on the back side 134 of theinternal surface 130. In this case, the propagating image illuminationis the image illumination that has undergone reflection at the upperface of the LOE (e.g., the face 104 in FIG. 14 or the face 14 in FIG. 1). The part of the of the propagating image illumination represented bythe light ray 118A impinges on a region of the internal surface 130having spaces 35 between the portions 34 of reflective material, and istherefore transmitted by the internal surface 130 as light ray 120(i.e., the light ray 118A passes through the internal surface 130 fromthe back side 134 to the front side 132, due to the spaces 35 beingtransparent). The part of the propagating image illumination representedby the light ray 118B passes through the back side 134 of the internalsurface 130 and impinges on a region of the internal surface 130 havingthe reflective material so as to be reflected by one of the portions 34of reflective material as reflected light ray 122. This light ray 122,as discussed above, can undergo additional reflections at the faces ofthe LOE and ultimately be reflected at the front side of one of theinternal surfaces so as to be reflected out of the LOE as a ghost lightray.

In order to combat these undesired reflections, embodiments of thepresent invention provide a coating of reflection suppressing materialapplied between the portions of reflective material and the front sideof the internal surfaces. FIGS. 16A and 16B show the reflectionsuppressing material and its effect on propagating image illumination.Similar to as in FIGS. 15A and 15B, FIGS. 16A and 16B are not drawn toscale for clarity of illustration.

Looking first at FIG. 16A, a coating of reflection suppressing material,designated as portions 150, is deployed between the portions 34 ofreflective material and the front side 132 of the internal surface 130.If the coating 30 is implemented using a planar base surface 32 (e.g.,thin-film), the portions 150 can be deposited directly on the surface32, and the portions 34 can then be deposited on the portions 150.Preferably, the portions of the reflection suppressing material arearranged in the same pattern configuration as the portions of reflectivematerial, such that the portions 34 and 150 are identical in size,shape, and number. As can be seen in FIG. 16A, the reflectionsuppressing material has little to no effect on propagating imageillumination that is incident to the front side 132 of the internalsurface 130. Similar to as discussed above with reference to FIG. 15A,the part of the propagating image illumination represented by the lightray 140A impinges on a region of the internal surface 130 having thereflective material so as to be reflected by one of the portions 34 ofreflective material as reflected light ray 142. The part of the of thepropagating image illumination represented by the light ray 140Bimpinges on a region of the internal surface 130 having spaces 35between the portions 34 of reflective material, and is transmitted bythe internal surface 130 as light ray 142.

Turning now to FIG. 16B, there is shown how the internal surface 130with the reflection suppressing material handles propagating imageillumination 118 that impinges on the back side 134 of the internalsurface 130. Similar to as discussed above with reference to FIG. 15B,the part of the of the propagating image illumination represented by thelight ray 118A impinges on a region of the internal surface 130 havingspaces 35 between the portions 34 of reflective material, and istherefore transmitted by the internal surface 130 as light ray 120.However, unlike the configuration illustrated in FIG. 15B, the part ofthe propagating image illumination represented by the light ray 118Bpasses through the back side 134 of the internal surface 130 andimpinges on a region of the internal surface 130 that has a portion 150of the reflection suppressing material. The reflection suppressingmaterial prevents the backside reflection of the light ray 118B, andtherefore no undesired reflection of propagating image illuminationoccurs.

The reflection suppressing material can be implemented in various ways.In one non-limiting example, the reflection suppressing material isimplemented as an amount of black absorbing paint, which absorbsincident light. In another non-limiting example, the reflectionsuppressing material is implemented as an amount of light scatteringmaterial (such as a diffusive material), that scatters incident light inmultiple directions at intensities that are orders of magnitude smallerthan the intensity of the incident light. As a result, any scatteredlight that continues propagating through the LOE and is reflected by asubsequent internal surface will have an intensity that is generally toolow to be noticeable to the observer.

The reflection suppressing material is preferably deposited between thereflective material and the front side of the internal surfaces duringmanufacturing of the LOE. The LOE, with embedded internal surfaces, ispreferably constructed by forming a stack of transparent plates (e.g.,glass plates) bonded together with suitable coatings at theirinterfaces. The boding is typically performed using optical cement. Thecoatings can include the patterned reflective coatings and/or dielectriccoatings, all as described above. The coatings can be built up in layerson thin-films or thin-substrates (e.g., base surface 32), which areapplied at the interfaces between the transparent plates prior tobonding the plates together. Alternatively, the coatings can be built updirectly on the transparent plates prior to bonding the plates together,such that the transparent plates serve as the base surface 32. Whenemploying a reflection suppressing material to reduce ghost images,layers of the reflection suppressing material can be built-up in apattern (either directly on the transparent plates or on the thin-filmor thin-substrate), with the layers of the pattern reflective materialthen built-up on the reflection suppressing material, therebysandwiching the reflection suppressing material between the transparentplate and the reflective material.

Once the stack of transparent plates is bonded together, withappropriate coatings (and preferably reflection suppressing material) atthe interfaces, the stack is cut (i.e., sliced) at an appropriate angle(corresponding to the desired oblique angle at which the internalsurfaces are to be deployed) to form the LOE with partially reflectiveinternal surfaces embedded between parallel major external surfaces(i.e., faces). The slicing at the appropriate angle is referred to as“diagonal cutting” or “diagonal slicing”. The major external surfaces ofthe LOE are then polished to increase optical quality at the majorexternal surfaces. In embodiments in which the LOE uses a coupling-inreflector as the optical coupling-in configuration, similar steps can beperformed in order to produce a substrate having an embedded coupling-inreflector.

Although the polishing process has the desired effect of increasingoptical quality at the parallel faces of the LOE, the polishing processmay, in certain instances, create blemishes at interface regions betweenthe LOE substrate and the internal surface that can negatively affectoptical performance and image quality at the LOE output. One type ofblemish that can be caused by the polishing process is an indentation inone or both of the parallel faces of the LOE at the interface regionbetween the internal surface and the parallel faces of the substrate.Such a blemish is illustrated schematically in FIG. 17 (not drawn toscale), which shows a section of an LOE 200 having parallel faces 202,204 with an internal partially reflective surface 206 deployed obliqueto the faces 202, 204. Although not shown in the drawing, additionalinternal partially reflective surfaces are deployed within the LOE 200,parallel to the internal surface 206.

The internal surface 206 includes two opposing ends 208 a, 208 b (i.e.,starting and stopping ends) at corresponding end regions 210 a, 210 bthat are respectively associated with the faces 202, 204. The faces 202,204 and the respective end regions 210 a, 210 b (and in particular therespective ends 208 a, 208 b) define interface regions 212 a, 212 b(designated by the dashed circles) between the internal surface 206 andthe LOE substrate. An indentation 214 is formed, for example as a resultof the polishing process, in one of the faces 202 at the correspondinginterface regions 212 a (but can be formed in both faces, i.e., at bothinterface regions 212 a, 212 b). The indentation 214 is generally formedas a dent, depression, pit, cavity, or crevice in the face of the LOE,which causes a portion (albeit a small portion) of the face 202 toprotrude inward into the interior section of the LOE 200 in which theinternal surfaces are deployed. The protruding portion (i.e., theprotrusion) is generally designated 216 in FIG. 17 .

Typically, the indentation 214 is formed as a result of the polishingprocess due to pressure applied during polishing at the interfaceregions 212 a, 212 b, which may have reduced structural integritycompared with the remaining portions of the faces 202, 204. Othersources besides polishing may cause the formation of the indentation214, for example, mishandling (e.g., dropping) of the LOE.

As a result of the indentation 214, image illumination that propagatesat or near the interface region 212 a may undergo scattering by theprotrusion 216. This is illustrated schematically in FIG. 18 , whereimage illumination 218 (schematically represented by light ray 218) istransmitted by the internal surface 206, and undergoes internalreflection at the face 204 so as to generate reflected light ray 220(which is also part of the image illumination). The light ray 220 isincident to the face 202 at or near the protrusion 216 so as to impingeon the protrusion 216, causing the incident light ray 220 to bereflected in multiple directions (i.e., scattered) by the protrusion216, schematically represented by scattered light rays 222 a-222 c. Thelight rays are scattered in various directions due to the varyingsurface profile of the protrusion 216. These scattered light rays 222a-222 b are undesired reflections, and can propagate through the LOE 200so as to be reflected by one of the subsequent internal surfaces atunwanted angles, resulting in ghost images at the eye of the observer,similar to the light ray 122 discussed above with reference to FIG. 15B.

Referring now to FIG. 19 , there is shown a method for combating thescattering effects caused by the indentation 214 by coating a portion ofthe face 202 that includes the indentation 214 with a light absorbingmaterial. In particular, an amount of a light absorbing material 224 isdeposited on the portion of the face 202 that includes the indentation214. Preferably, the amount of the light absorbing material 224 that islocated in the indentation 214 is sufficient to fill the indentation 214to at least the level of the unblemished portions of the face 202. Inone non-limiting example, the light absorbing material 224 isimplemented as black absorbing paint that is applied to the face 202 inan amount that is sufficient to fill the indentation 214. The face 202is preferably then polished to remove any excess light absorbingmaterial from the face 202, such that only the light absorbing materiallocated in the indentation 214 remains, and the level of the lightabsorbing material 224 in the indentation 214 is flush with theunblemished portions of the face 202

The effect of the light absorbing material 224 on propagating imageillumination is also illustrated in FIG. 19 . Similar to as discussedabove with reference to FIG. 18 , the light ray 218 is transmitted bythe internal surface 206, and undergoes internal reflection at the face204 so as to generate reflected light ray 220. However, the light ray220, upon impinging on the protrusion 216, is absorbed by the lightabsorbing material 224, thereby preventing scattering of light by theprotrusion 216.

The light absorbing material can be applied at any of the interfaceregions between the internal surfaces and the LOE substrate at whichsuch indentations are present and then polished off as described above.For example, the light absorbing material can be applied to anindentation formed in the interface regions 212 b. In addition, whenusing a coupling-in reflector (i.e., an internal reflecting surface) asthe optical coupling-in configuration, indentations may form at theinterface regions between the internal reflecting surface and the LOEsubstrate during the polishing process. Here too an amount of lightabsorbing material can be applied at interface regions between theinternal reflecting surface and the LOE substrate to combat scatteringeffects induced by the indentations.

Although the scattering reduction by use of a light absorbing materialapplied to blemishes at external regions of an LOE has been describedwithin the context an LOE in which light propagates in one dimension andis coupled-out by internal surfaces so as to perform aperture expansionin one dimension, the light absorbing material can similarly be appliedto blemishes on external regions or portions of optical waveguides thatperform aperture expansion in two dimensions, such as the opticalwaveguides that perform guided-to-guided image propagation describedwith reference to FIGS. 12A, 12B and 13 . These blemishes can includeindentations formed at interface regions between the various sets offacets (e.g., facets 58, 64, 76, 86) and the corresponding faces (e.g.,faces 52 a, 52 b, 54 a, 54 b, 62 a, 62 b, 72 a, 72 b, 74 a, 74 b, 82 a,82 b, 84 a, 84 b).

The light absorbing material can also be used to fix blemishes in theform of scratches on the faces of the optical waveguides and/or chippedcorners or edges of the optical waveguides. For example, consider theoptical waveguide 50 of FIGS. 12A and 12B, reproduced in FIG. 20 . Here,a portion of the corner/edge that is formed by the faces 52 a, 54 a hasbeen chipped off (due to, for example, mishandling of the opticalwaveguide 50), resulting in blemish 230. Light propagating through theoptical waveguide 50 by four-fold internal reflection that impinges onthe region of the blemish 230 will be scattered or undergo reflectionsin undesired directions. As shown in FIG. 21 , an amount of lightabsorbing material 224 can be applied at the blemish 230 so as toprevent the scattering effect. In FIG. 21 , the amount of lightabsorbing material located at the blemish is sufficient so as to restorethe rectangular cross-section of the optical waveguide 50. However, alesser amount of light absorbing material may be applied to blemisheswhich do not restore the optical waveguide to its unblemished structure.The light absorbing material can equally be applied to fill scratches atthe faces of the optical waveguides (for both one-dimensional andtwo-dimensional aperture expanding optical devices), e.g., for any ofthe optical waveguides 10, 50, 60, 70, 80, 100.

It is noted that certain aspects of the present invention describedherein can be used to advantage independently of other aspects of thepresent invention. For example, the complementary coating methodologies,used either with or without interleaved sets of facets, can be used toadvantage separately from the blemish mending techniques. Moreover, theblemish mending techniques can be applied to LOEs or optical waveguides(performing one-dimensional or two-dimensional aperture expansion)having otherwise conventional coating architectures.

Although only the LOE and optical waveguide structures have beenillustrated in the drawings, it will be understood that the various LOEsand optical waveguides described herein are intended for use as part ofa display, typically a head-up display (HUD), which is preferably anear-eye display (NED), such as a head-mounted display (HMD) orglasses-frame supported display, for providing an image to an eye of anobserver. In certain preferred embodiments, the display is part of anaugmented reality (AR) display system, in which the image provided tothe eye of the observer is overlaid on external “real-world” scenery. Inother embodiments, the display is part of a virtual reality (VR) displaysystem, in which only the image provided by the LOE/optical waveguide isviewable to the observer. In all such cases, the display preferablyincludes an image projector of small form factor that generates acollimated image, which is optically coupled to the LOE/opticalwaveguide so as to introduce the collimated image into the LOE/opticalwaveguide via an optical coupling-in configuration (e.g., thecoupling-in reflector 22, coupling prism, etc.) so as to propagate byinternal reflection within the LOE/optical waveguide and gradually becoupled out by the internal selectively reflective surface.

Examples of suitable image projectors for projecting illumination (i.e.,light) corresponding to (i.e., indicative of) a collimated image, forexample, employing an illumination source, a spatial light modulatorsuch as a liquid crystal on silicon (LCoS) chip, and collimating optics,typically all arranged on surfaces of one or more polarization selectivebeamsplitter (PBS) cube or other prism arrangement, are well known inthe art.

It is noted that when used within the context of AR systems, applicationof small amounts of the light absorbing material on blemishes atexternal portions of the optical waveguides may also provide benefits ofreducing or suppressing scattering of light from external scenery.

When discussing polarization properties of image illumination andcoatings, it is noted that for each instance where a particularpolarized wave path has been followed in the examples described herein,the polarizations are interchangeable, whereby, for example, on alteringpolarization selective properties of the coatings, each mention ofp-polarized light could be replaced by s-polarized light, and viceversa.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

As used herein, the singular form, “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration”. Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

To the extent that the appended claims have been drafted withoutmultiple dependencies, this has been done only to accommodate formalrequirements in jurisdictions which do not allow such multipledependencies. It should be noted that all possible combinations offeatures which would be implied by rendering the claims multiplydependent are explicitly envisaged and should be considered part of theinvention.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

What is claimed is:
 1. An optical device, comprising: alight-transmitting substrate having at least two parallel major externalsurfaces for guiding light indicative of a collimated image by internalreflection at the major external surfaces; a first set of mutuallyparallel internal surfaces deployed within the substrate oblique to theexternal surfaces; and a second set of mutually parallel internalsurfaces deployed within the substrate parallel to, interleaved with andin overlapping relation with, the first set of internal surfaces, atleast part of each of the internal surfaces of the first set including afirst coating having a first reflection characteristic so as to be atleast partially reflective to at least a first subset of components ofincident light, and at least part of each of the internal surfaces ofthe second set including a second coating having a second reflectioncharacteristic, that is complementary to the first reflectioncharacteristic, so as to be at least partially reflective to at least asecond subset of components of incident light, such that the sets ofinternal surfaces cooperate to reflect all components of light from thefirst and second subsets.
 2. The optical device of claim 1, wherein thefirst subset of components includes light corresponding to a firstcolor, and wherein the second subset of components includes lightcorresponding to a second color.
 3. The optical device of claim 1,wherein the first subset of components includes light having a firstpolarization direction, and wherein the second subset of componentsincludes light having a second polarization direction.
 4. The opticaldevice of claim 1, wherein at least one of the first or second coatingsincludes a material selected from the group consisting of: a structuralpolarizer, a dielectric coating, and a metallic coating.
 5. The opticaldevice of claim 1, wherein the first coating is configured to: reflectlight having wavelengths corresponding to a first color with a firstreflection efficiency, reflect light having wavelengths corresponding toa second color with a second reflection efficiency, and reflect lighthaving wavelengths corresponding to a third color with a thirdreflection efficiency less than the first reflection efficiency, andwherein the second coating is configured to reflect light havingwavelengths corresponding to the first color with a reflectionefficiency that is greater than the third reflection efficiency, suchthat the combined reflection efficiency of the third color by the firstand second coatings is greater than or equal to the first reflectionefficiency.
 6. The optical device of claim 5, wherein the secondreflection efficiency is less than the first reflection efficiency, andwherein the second coating is configured to reflect light havingwavelengths corresponding to the second color with a reflectionefficiency that is greater than the second reflection efficiency, suchthat the combined reflection efficiency of the second color by the firstand second coatings is greater than or equal to the first reflectionefficiency.
 7. The optical device of claim 6, wherein the second coatingis configured to reflect light having wavelengths corresponding to thefirst color with a reflection efficiency that is approximately equal tothe first reflection efficiency.
 8. The optical device of claim 1,wherein the first coating is configured to: reflect light havingwavelengths corresponding to a first color with a first reflectionefficiency, reflect light having wavelengths corresponding to a secondcolor with a second reflection efficiency less than the first reflectionefficiency, and reflect light having wavelengths corresponding to athird color with a third reflection efficiency less than the firstreflection efficiency, and wherein the second coating is configured to:reflect light having wavelengths corresponding to the first color at areflection efficiency greater than the second and third reflectionefficiencies, reflect light having wavelengths corresponding to thesecond color at a reflection efficiency greater than the second andthird reflection efficiencies, and reflect light having wavelengthscorresponding to the third color at a reflection efficiency greater thanthe second and third reflection efficiencies.
 9. The optical device ofclaim 1, wherein the first coating includes a patterned coatingcomprising a number of portions of a reflective material arranged oneach of the internal surfaces of the first set in a prescribed pattern.10. The optical device of claim 9, wherein each portion of thereflective material has a circular shape or an oblong shape in a planeof the internal surfaces.
 11. The optical device of claim 9, wherein thereflective material is a dielectric material or a metallic material. 12.The optical device of claim 9, wherein spaces formed between theportions of the reflective material are transparent.
 13. The opticaldevice of claim 9, wherein a second reflective material is deployed onthe internal surfaces in spaces formed between the portions of thereflective material.
 14. The optical device of claim 13, wherein thesecond reflective material includes a dielectric material.
 15. Theoptical device of claim 13, wherein the second reflective material isarranged on the internal surfaces in a prescribed pattern.
 16. Theoptical device of claim 9, wherein at least one of the number ofportions or a size of the portions on the internal surfaces of the firstset increases with respect to a primary direction of propagation oflight through the substrate.
 17. The optical device of claim 9, furthercomprising an amount of a light reflection suppressing material deployedbetween the reflective material and at least part of the internalsurfaces of the first set.
 18. The optical device of claim 1, whereinthe first coating is deployed on a first portion of each of the internalsurfaces of the first set, and wherein the second coating is deployed ona second portion of each of the internal surfaces of the first set, andwherein the second coating is deployed on a first portion of each of theinternal surfaces of the second set, and wherein the first coating isdeployed on a second portion of each of the internal surfaces of thesecond set, and wherein the first and second portions of the internalsurfaces of the first set are non-overlapping portions, and wherein thefirst and second portions of the internal surfaces of the second set arenon-overlapping portions.
 19. An optical device, comprising: alight-transmitting substrate having at least two parallel major externalsurfaces for guiding light indicative of a collimated image by internalreflection at the major external surfaces; and a plurality of mutuallyparallel internal surfaces deployed within the substrate oblique to theexternal surfaces, at least part of a first subset of the internalsurfaces comprising a patterned coating that includes a number ofportions of a reflective material arranged on the internal surfaces ofthe first subset in a prescribed pattern, the patterned coating being atleast partially reflective to at least a first subset of components ofincident light, a second subset of the internal surfaces being at leastpartially reflective to at least a second subset of components ofincident light, and the internal surfaces of the first subset being inoverlapping relation with the internal surfaces of the second subsetsuch that the subsets of internal surfaces cooperate to reflect allcomponents of light from the first and second subsets.
 20. The opticaldevice of claim 19, wherein spaces formed between the portions of thereflective material are transparent.
 21. The optical device of claim 19,wherein a second reflective material is deployed in spaces formedbetween the portions of the reflective material.
 22. The optical deviceof claim 21, wherein the second reflective material includes adielectric material.
 23. The optical device of claim 21, wherein thesecond reflective material is arranged on the internal surfaces of thefirst subset in a prescribed pattern.
 24. The optical device of claim19, wherein the internal surfaces of the first subset are interleavedwith the internal surfaces of the second subset.
 25. The optical deviceof claim 19, wherein surfaces of the first subset of internal surfacesare coplanar with surfaces of the second subset of internal surfaces.